Multi-Wavelength LIDAR System

ABSTRACT

A multi-wavelength LIDAR system includes a first laser source that generates a first optical beam having a first wavelength and a second laser source that generates a second optical beam having a second wavelength. An optical element projects the first optical beam to form a first beam profile at a target plane and projects the second optical beam to form a second beam profile at the target plane. An optical receiver generates a first wavelength signal corresponding to the received reflected portion of the first beam profile and generates a second wavelength signal corresponding to the reflected portion of the second beam profile at the target plane. A controller generates a measurement point cloud from the first and second wavelength signals, wherein an angular resolution of the measurement point cloud depends on a relative position of the first and second beam profiles at the target plane.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a non-provisional application of U.S.Provisional Patent Application No. 62/326,576, entitled“Multi-Wavelength Lidar System” filed on Apr. 22, 2016 and U.S.Provisional Patent Application No. 62/396,295, entitled “WDM LidarSystem” filed on Sep. 19, 2016. The entire contents of U.S. ProvisionalPatent Application No. 62/326,576 and U.S. Provisional PatentApplication No. 62/396,295 are herein incorporated by reference.

The section headings used herein are for organizational purposes onlyand should not to be construed as limiting the subject matter describedin the present application in any way.

INTRODUCTION

Autonomous (self-driving) and semi-autonomous automobiles use acombination of different sensors and technologies such as radar,image-recognition cameras, and sonar for detection and location ofsurrounding objects. These sensors enable a host of improvements indriver safety including collision warning, automatic-emergency braking,lane-departure warning, lane-keeping assistance, adaptive cruisecontrol, and piloted driving. Among these sensor technologies, lightdetection and ranging (LIDAR) systems are one of the most criticalenabling real-time measurements of object distances.

BRIEF DESCRIPTION OF THE DRAWINGS

The present teaching, in accordance with preferred and exemplaryembodiments, together with further advantages thereof, is moreparticularly described in the following detailed description, taken inconjunction with the accompanying drawings. The skilled person in theart will understand that the drawings, described below, are forillustration purposes only. The drawings are not necessarily to scale,emphasis instead generally being placed upon illustrating principles ofthe teaching. The drawings are not intended to limit the scope of theApplicant's teaching in any way.

FIG. 1 illustrates the operation of a LIDAR system of the presentteaching implemented in a vehicle.

FIG. 2 illustrates a schematic diagram representing two-dimensionalfield-of-view and range requirements of a typical LIDAR sensing systemfor the surroundings of an automobile.

FIG. 3 illustrates an embodiment of a multi-module LIDAR sensor systemof the present teaching.

FIG. 4 illustrates a table presenting the horizontal angular resolutionas a function of distance for an embodiment of a LIDAR sensor system ofthe present teaching.

FIG. 5 illustrates a table presenting the advertised performance ofVelodyne LiDAR™ systems.

FIG. 6 illustrates a table presenting the performance specifications ofan embodiment of a LIDAR system of the present teaching.

FIG. 7 illustrates an embodiment of a multi-wavelength LIDAR systemusing two lasers of the present teaching.

FIG. 8 illustrates an embodiment of an illuminator for amulti-wavelength LIDAR system using two lasers and a wavelength combinerof the present teaching.

FIG. 9 illustrates an embodiment a receiver that uses multiple-receiversfor different wavelengths of the present teaching.

FIG. 10 illustrates an embodiment of a simple coding scheme for amulti-wavelength LIDAR system according to the present teaching.

FIG. 11 illustrates a drawing of a prior art cluster VCSEL device thatcomprises a cluster formed by twenty-one individual apertures.

FIG. 12A illustrates a chip comprising multiple cluster VCSEL devicesarranged individually.

FIG. 12B illustrates a chip comprising multiple cluster VCSEL devicesarranged in bars.

FIG. 12C illustrates a top-view of an anode metal contact pad of a chipcomprising multiple cluster VCSEL devices of the present teaching.

FIG. 12D illustrates a bottom-view of a cathode metal contact pad of thechip comprising the multiple cluster VCSEL devices illustrated in FIG.12C.

FIG. 13 illustrates an embodiment of a multi-emitter laser source for amulti-wavelength LIDAR system of the present teaching.

FIG. 14 illustrates a diagram of the cross-section of an embodiment ofan illuminator for a multi-wavelength LIDAR system of the presentteaching.

FIG. 15A illustrates a measurement point cloud for an embodiment of asingle-wavelength 2D multi-emitter laser source illumination of thepresent teaching.

FIG. 15B illustrates a measurement point cloud for an embodiment of atwo-wavelength 2D multi-emitter laser source illumination of the presentteaching.

FIG. 16 illustrates a measurement point cloud for an embodiment ofanother two-wavelength LIDAR of the present teaching.

FIG. 17 illustrates a measurement point cloud for an embodiment of athree-wavelength LIDAR of the present teaching.

FIG. 18 illustrates an embodiment of an illuminator for amulti-wavelength LIDAR system of the present teaching.

FIG. 19 illustrates a measurement point cloud that can be generated withthe illuminator embodiment of FIG. 18 .

FIG. 20 illustrates an embodiment of a measurement point cloud generatedusing two multi-emitter sources in a multi-wavelength LIDARconfiguration of the present teaching.

FIG. 21 illustrates a measurement point cloud generated by an embodimentof a multi-wavelength LIDAR where the density of measurement points atone wavelength across the full field-of-view is half that of a secondwavelength according to the present teaching.

FIG. 22 illustrates an embodiment of a measurement point cloud of amulti-wavelength LIDAR utilizing four wavelengths of the presentteaching.

FIG. 23A illustrates a VCSEL array layout for an embodiment of amulti-wavelength LIDAR where the angular resolution changes in aparticular direction according to the present teaching.

FIG. 23B illustrates a measurement point cloud generated by anembodiment of a multi-wavelength LIDAR with varying angular resolutionaccording to the present teaching.

FIG. 24A illustrates an embodiment of a multi-mode multi-emitter VCSELlaser source of the present teaching.

FIG. 24B illustrates a cross section of a projected beam profile of themulti-mode multi-emitter VCSEL laser source of FIG. 24A.

FIG. 24C illustrates the projection angles for the multi-modemulti-emitter VCSEL laser source of FIG. 24A.

FIG. 25 illustrates an embodiment of a VCSEL array comprising a deviceused to measure temperature located in close proximity to the VCSELdevices according to the present teaching.

FIG. 26 illustrates an embodiment of a VCSEL array comprising an activethermal control device to control the VCSEL array temperature accordingto the present teaching.

FIG. 27 illustrates an embodiment of a temperature-controlled VCSELarray comprising a thermo-electric cooler (TEC) for heating and coolingthe VCSELs according to the present teaching.

FIG. 28A illustrates a top-view of an embodiment of a compact VCSELlaser driver assembly of the present teaching.

FIG. 28B illustrates a side-view of the embodiment of the compact VCSELlaser driver assembly of FIG. 28A.

FIG. 28C illustrates a bottom-view of the embodiment of the compactVCSEL laser driver assembly of FIG. 28A.

FIG. 29 illustrates another embodiment of a compact VCSEL laser driverassembly for multi-wavelength LIDAR of the present teaching.

FIG. 30 illustrates a system block diagram of an embodiment of a compactVCSEL laser driver assembly for multi-wavelength LIDAR of the presentteaching.

FIG. 31 illustrates an embodiment of a multi-wavelength optical powermonitor for multi-element multi-wavelength LIDAR systems of the presentteaching.

FIG. 32 illustrates an embodiment of a VCSEL cluster layout comprisinglaser apertures emitting different wavelengths according to the presentteaching.

FIG. 33 illustrates an embodiment of an illuminator comprising the VCSELcluster layout of FIG. 32 .

DESCRIPTION OF VARIOUS EMBODIMENTS

The present teaching will now be described in more detail with referenceto exemplary embodiments thereof as shown in the accompanying drawings.While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives, modifications and equivalents, as willbe appreciated by those of skill in the art. Those of ordinary skill inthe art having access to the teaching herein will recognize additionalimplementations, modifications, and embodiments, as well as other fieldsof use, which are within the scope of the present disclosure asdescribed herein.

Reference in the specification to “one embodiment” or “an embodiment”means that a particular feature, structure, or characteristic describedin connection with the embodiment is included in at least one embodimentof the teaching. The appearances of the phrase “in one embodiment” invarious places in the specification are not necessarily all referring tothe same embodiment.

It should be understood that the individual steps of the methods of thepresent teachings can be performed in any order and/or simultaneously aslong as the teaching remains operable. Furthermore, it should beunderstood that the apparatus and methods of the present teachings caninclude any number or all of the described embodiments as long as theteaching remains operable.

The present teaching relates to Light Detection and Ranging Systems(LIDAR) that measure distances to various objects or targets thatreflect and/or scatter light. In particular, the present teachingrelates to LIDAR systems that advantageously use multiple wavelengths oflight to improve performance and that may also reduce size, cost, andcomplexity compared to prior art LIDAR systems.

Systems of the present teaching may use light sources that includesingle emitters and/or multiple emitters. For example, light sourcesthat use a single element VCSEL or a single edge-emitting laser devicewould be considered single emitters. Light sources that use multipleVCSEL elements or multiple edge-emitting laser sources arranged on oneor more substrates are considered multiple emitter sources. Themulti-element emitters may be configured in various arrayconfigurations, including one-dimensional and two-dimensional arrays.The descriptions below refer to various embodiments with single-emittersources and/or multi-emitter laser sources. However, it will be apparentto those familiar with the art that the features of particularembodiments of LIDAR systems of the present teaching should not beconsidered limited to either single-emitter and/or multi-emitter lasersources, but rather should be more broadly construed to apply to bothsingle-emitter and/or multi-emitter laser sources as consistent with thepresent teaching.

FIG. 1 illustrates the operation of a LIDAR system 100 of the presentteaching implemented in a vehicle. The LIDAR system 100 includes a laserprojector, also referred to as an illuminator, that projects light beams102 generated by a light source toward a target scene and a receiverthat receives the light 104 that reflects of an object, shown as aperson 106, in that target scene. LIDAR systems typically also include acontroller that computes the distance information about the object 106from the reflected light, and an element that can scan or provide aparticular pattern of the light that may be a static pattern across adesired range and field-of-view (FOV). The receiver and controller areused to convert the received signal light into measurements thatrepresent a pointwise 3D map of the surrounding environment that fallswithin the LIDAR system range and FOV. In various embodiments, thecontroller can be a simple electrical circuit or a more complicatedprocessor, depending on the particular application.

The laser source and optical beam projection means that form theilluminator and the receiver may be located on the front side of avehicle 108. A person 106, and/or another object, such as a car or lightpole, will provide light reflected from the source back to the receiver,and a range, or distance, to that object is determined. As is known inthe art, a LIDAR receiver calculates range information based ontime-of-flight measurements of light pulses emitted from the lightsource. In addition, known information about the optical beam profilethat illuminates the scene in a target plane associated with aparticular range and based on the particular design of the source andprojector system is used to determine location information about thereflecting surface, thereby generating a complete x,y,z, orthree-dimensional picture of the scene. In other words, the pointwise 3Dmap of the surrounding environment represents a collection ofmeasurement data that indicates position information from all thesurfaces that reflect the illumination from the source to the receiverwithin the field-of-view of the LIDAR system. In this way, a 3Drepresentation of objects in the field-of-view of the LIDAR system isobtained. The pointwise 3D data map may also be referred to as ameasurement point cloud.

FIG. 2 illustrates a schematic diagram representing two-dimensionalfield-of-view and range requirements of a typical surroundings sensingLIDAR system 200 for an automobile 202. For example, an adaptive cruisecontrol function may require a field-of-view and range 204 with a narrowfield-of-view, but a longer-distance range requirement, compared to theside looking “surround view” field-of-view and range 206. In general,sensor functions on an automobile may be enabled by a combination ofLIDAR, radar, cameras, and ultrasonic sensors. The combining of thesesensor data generate information about the surrounding environment isoften referred to as “sensor fusion”.

Although the present teaching describes LIDAR systems in the context ofautomotive vehicles, where LIDAR is widely used for autonomous, orself-driving, or driver-assisted, vehicles, it should be understood thatthe embodiments may be applicable to any vehicle. Other types ofvehicles might include robots, tractors, trucks, airplanes, drones,boats, ships, and others. The present teachings are also applicable tovarious stationary applications. For example, in high density,metropolitan areas, LIDAR could be employed to monitor traffic, bothvehicular and pedestrian. We can expect to see LIDAR deployed in manydifferent applications as the cost of the LIDAR systems come down overtime.

FIG. 3 illustrates a sensor system 300 for an automobile 302 of thepresent teaching. There are six separate LIDAR modules 304 that operatewith various ranges and FOVs, illustrated by the 2D wedges in thediagram. Four of the LIDAR modules 304 are predominantly side-view, andeach have a range and FOV 306 of approximately 120 degrees. The forwardmodule is shown with a range and FOV 308 that has the narrowest FOV andlongest distance range.

Some prior art LIDAR system designs for automobile sensing disclose thata LIDAR system should be able to resolve a pedestrian walking in frontof the car at a distance of in order to provide adequate time to avoid acollision. The pedestrian cross section would roughly be 280 mm, so theangular resolution required for the LIDAR system needs to beapproximately 0.5 degrees. See, for example, U.S. Pat. No. 7,544,945entitled “Vertical Cavity Surface Emitting Laser (VCSEL) Array LaserScanner.”

However, one feature of the present teaching is the recognition thatautomotive applications can demand a much higher angular resolution,which in some application is much less than 0.5 degree. Higher angularresolution can enable some form of image identification. For example, ahigher resolution would allow the sensor system to be able todistinguish between a pedestrian and a light pole, or between twomotorcycles and a car. As such, a target specification of 0.1-degreeangular resolution at 100-m range may be required. The angularresolution could be less for shorter distances.

FIG. 4 illustrates a table 400 providing the horizontal angularresolution as a function of distance for one embodiment of a LIDARsensor system of the present teaching. The data presented in the table400 assumes the physical distance measured is the important parameter.

Future vehicles will adopt multiple low-cost LIDAR sensors in order tocover the complete 360-degree FOV. It also appears there may be adifferent LIDAR requirement of distance for side-view and blind spotcompared with the LIDAR requirement of distance for looking directly infront of the vehicle. For a solid-state LIDAR system that does notrotate, it's understood that the FOV must be less than 180 degrees.Prior art LIDAR systems manufactured by Innoviz Technologies, forexample, advertise a 100 degree horizontal (H)×25 degree vertical (V)FOV combined with a 0.1 degree angular resolution. Prior art LIDARsystems manufactured by Quanergy Systems advertise a 120 degreehorizontal FOV with angular resolution of less than degree. QuanergySystems' LIDAR uses an optical phased array as a transmitter, which cansteer pulses of light by shifting the phase of a laser pulse as it'sprojected through the array. One disadvantage of using phased arrays isthe presence of side lobes that can impact the illumination pattern atthe target. The solid-state laser source LIDAR systems of the presentteaching do not produce side lobes associated with phased arrayapproaches. Another limitation of prior art systems using asingle-wavelength of light, is the speed of light. Travel time for 200meters is microseconds, limiting a LIDAR system to fire the laser aboutevery microsecond or so, depending on overhead. The laser pulse rate istypically not higher than this one MHz rate, within the FOV of thedetector, so that the system is capable of identifying the return pulsefor each measurement point without ambiguity.

Some LIDAR systems utilize multiple lasers in the illuminator to helpimprove system performance, such as angular resolution, refresh rate,and FOV. However, prior art multi-laser-source compact LIDAR systems,such as those manufactured by Velodyne LiDAR™ systems, cannot achieve anarbitrarily fine angular resolution, a wide FOV, and a high refresh ratewith a single unit using thirty-two lasers.

FIG. 5 illustrates a table 500 presenting the advertised performance ofa LiDAR™ system commercially available from Velodyne. These Velodynesystems rotate, and so have a horizontal field-of-view of 360-degree. Inthe vertical direction, the 32-lasers are arranged to provide anadvertised 40-degree field-of-view, with a uniform angular resolution of1.25 degree. It should be clear that with a rotating system, the fullfield-of-view can only be refreshed at the rate of rotation. As well,all 32-lasers are arranged in a single vertical line, such that thesystem essentially is pointing in a single horizontal direction when notrotating. Such a system typically is operated with a rotation rate of 10Hz, to provide an angular resolution of ˜0.16 degrees. Slower rotationspeeds can be used to achieve a finer angular resolution, but at theexpense of refresh rate. If an object moves significantly in the time ittakes the rotating system to complete one revolution, the vehicle maynot be able to respond quick enough to avoid or steer appropriatelyaround the object. A system that can look in any direction, and at anytime, is desirable in order to be able to refresh the field-of-view at ahigher rate. In practice, multiple rotating LIDAR systems have been usedon a single automobile, as many as four LIDAR systems in some instances,to be able to provide a full 360 degree surround view with adequaterefresh rate and fine angular resolution.

One feature of the multi-wavelength LIDAR system of the present teachingis providing a relatively high refresh rate. The refresh rate is alsoknown as the frame rate. Some embodiments of the present teachingprovide a system refresh rate that is at least the same as the typicallow-cost CMOS camera system refresh rate of 30-Hz, and potentially ashigh as 1 kHz. To understand why a high refresh rate is important,consider an automobile traveling at 100 km/h. Under these conditions,the automobile will move about 3 meters in 0.1 seconds. So, if therefresh rate is only 10 Hz, objects in front of the car will movesignificantly in that time causing a significant loss of resolution. Fora LIDAR system of the present teaching, utilizing four wavelengths, with4,096 lasers being measured in one frame, and a pulse duration of 1microsecond, the refresh rate is 1 kHz for a single system. If multiplesystems are used to cover the complete 360-degree field-of-view, thenthe refresh rate would still be 1 kHz. This assumes a single pulse permeasurement. If we need multiple pulses per measurement, the refreshrate will be lower.

FIG. 6 illustrates a table 600 providing the specifications associatedwith various parameters of LIDAR systems of the present teaching. Thesystem specification in the table 600 assumes a solid-state system thatutilizes a multi-emitter 2D VCSEL array source. To achieve thisspecification, 4,096 individual lasers are required. This can beachieved, for example, by using an array of 32 lasers in the verticaldirection and 128 lasers in the horizontal direction. In a fourwavelength system, a 32×32 array is needed for each wavelength. Inembodiments in which the lasers are on a 250 micron pitch, an array of32×32 lasers would have an 8 mm×8 mm foot print. In other configurationsfor lower cost, smaller arrays can be used. In these configurations, thesystem can use fewer lasers to achieve arrays with a 4×4 mm foot print.The system specifications will improve with additional lasers. In thefuture, we expect we can obtain lasers on a 125-micron pitch allowing32×32 arrays in a 4×4 mm foot print.

It is desirable to realize module sizes for the LIDAR system of thepresent teaching that are less than 125 cm³, and/or modules with lessthan 2.5-inch dimension on a side. In some embodiments, the modulesinclude the illuminator and the receiver in the same module. In someembodiments, there is a separate module for the illuminator and thereceiver.

One feature of the present teaching is the illuminator includes lasersthat emit optical beams with individual, distinct wavelengths. FIG. 7illustrates an embodiment of a multi-wavelength LIDAR system 700 usingtwo lasers according to the present teaching. The first laser 702operates at a first wavelength, and the second laser 704 operates at asecond wavelength. The lasers may include integrated or separatecollimation optics (not shown) that form part of an optical projectionelement that is used to form a beam profile at various target planesacross the FOV and range of the LIDAR system. The illuminator 706 canalso include an optical device 708 that further shapes and projects theoptical beams to form particular beam profiles a target plane 710. Invarious embodiments, various types of optical devices can be used toform the projection element including, for example, one or more oflenses, diffractive optics, prisms, thin-film wavelength sensitivedevices, and partially reflecting mirrors.

A receiver 712 receives light reflected off the surface of objects atvarious target planes 710 in the FOV and range of the LIDAR system. Thereceiver 712 can distinguish light from the two wavelengths emitted bythe sources 702, 704. As such, reflected illumination from eachwavelength is processed separately. A controller 714 is used to processthe received light. The controller 714 provides LIDAR data at an output716. The complexity of the controller 714 depends on the particularembodiment of the LIDAR system. The controller 714 may be used tocontrol the laser sources 702, 704. In various embodiments, thecontroller 714 may comprise any or all of electrical circuits,electronic chips, microprocessors or computers. It is straightforward toadd N wavelengths of lasers to the embodiment shown in FIG. 7 . In someembodiments, there are additional optical elements that collimate thelight and provide the desired FOV.

A projection element is described herein as an element that collimatesor otherwise shapes and project a laser beam or multiple laser beams ina particular direction. A projection element may comprise one or moreoptical devices positioned in the path of the optical beams. Thesedevices and their positions, together with the initial shape and pathsof the beam or beams emitted from the laser source, produce the desiredbeam profile, which is a combination of beam shape and/or beam positionat a particular point in space.

In some embodiments, additional optical devices are used to project theoptical beams from the laser sources. FIG. 8 illustrates an embodimentof an illuminator 800 for a multi-wavelength LIDAR system using twosingle-emitter lasers 802, 804 and a wavelength combiner 806 of thepresent teaching. Two laser sources 802, 804 with different wavelengthsgenerate two optical beams 808, 810 on two paths. A wavelength combiner806 is used to combine the beams onto two parallel paths 812, 814. Insome embodiments, the parallel paths are offset, as illustrated in FIG.8 . In some embodiments, the parallel paths completely overlap. In someembodiments, the paths after the wavelength combiner 806 are notnecessarily parallel, but are arranged to produce a desired beam profileat target planes in the FOV and range of the LIDAR system. The use of awavelength combiner 806 allows additional flexibility in both thephysical layout of the system and the associated beam profiles that aregenerated by the LIDAR.

One feature of the present teaching is the ability to use differentwavelengths to produce different LIDAR FOV, range, and/or resolution ina compact system. The light beams at the two or more wavelengths may beable to share at least some of the same optical devices that form theprojection element, and yet still realize different beam profiles thatresult in a measurement point cloud that represents different rangeand/or FOV and/or resolution at each wavelength. For example, oneproblem with prior art LIDAR systems that use a signal wavelength isthat the launch power required to reach 100-meter range is so high thatfor close proximity reflections (e.g. a few meters) the receiversaturates. This means these prior art LIDAR systems are blind to nearobjects. This problem can be solved with a two-wavelength system, wherethe first wavelength is used for a 100-meter range, but the secondwavelength has a low power only meant for near-proximity measurements.Using the two-wavelengths, the measurements can be simultaneous becauseof the parallel operation capability. The extension to more than twowavelengths is straightforward.

Another feature of the present teaching is that lasers with additionalwavelengths can be added to perform other functions than LIDAR ranging.For example, additional lasers can be added to provide measurements ofthe orientation of optical devices within the LIDAR system. The lightfrom these sources at additional wavelength may serve a sole purpose toprovide angular measurement of the elements that project the opticalbeams and/or replicate or scan the optical beams. In some embodiments,MEMs devices are used to project beams, and it can be important to havedirect feedback of the mirror position. Another laser combined withappropriate receiver measurement system could provide direct anglemeasurement of mirror position. A natural extension of the aboveembodiments would be to use a plurality of lasers of the same wavelengthin each case, that is, either a 1D or 2D array of lasers of eachwavelength instead of a single laser of each wavelength.

Some embodiments of the multi-wavelength LIDAR system of the presentteaching use a receiver comprising multiple receivers that each detectlight of different wavelengths independently. FIG. 9 illustrates anembodiment a receiver 900 that uses multiple-receivers 902 for differentwavelengths of the present teaching. In this embodiment, the reflectedlight 904 reflected from an object at a target plane in the range andFOV of the LIDAR system is detected by two or more receivers 902. Eachreceiver 902 is capable to detect a single wavelength. Amulti-wavelength LIDAR system using this embodiment for the receiver canenable simultaneous operation of the different laser wavelengths inwhich the pulse signals from each wavelength overlapped in time.

In a practical implementation of a LIDAR system, there are physicallimits on the speed of the electronics and the optical-electricalbandwidth of various components. A multi-wavelength LIDAR implementationwhere the laser wavelengths are capable of being detected separately,and simultaneously, can substantially reduce the impact of thesephysical limits. This results in an overall higher performance LIDARsystem.

One feature of the multi-wavelength LIDAR systems of the presentteaching is the inclusion of optical performance monitoring in themodule. Optical performance monitoring within the LIDAR module may beimportant for a variety of reasons. For example, incorporating opticalpower monitoring inside the illuminator assembly can improvecalibration, performance, and reliability monitoring. Lasers degradewith lifetime and so it may be useful to monitor the laser output powerwithin the projector assembly itself, as the light is exiting theprojector, rather than just relying on the received optical signal afterthe light has been reflected from an external object. Also, monitoringthe temperature proximate to the VCSEL lasers may be useful to improvethe reliability and performance. Monitoring the temperature and powercan be used not only for diagnostics, but also for controlling thelasers during operation to improve performance and/or lifetime of thesystem.

Some embodiments of the performance monitor of the multi-wavelengthLIDAR system of the present teaching use optical power monitoring. It iswell known that monitoring of the laser output power from thetransmitter, and comparing it to an expected reference value, will allowdetection of degradation in the performance of the optical transmitter,either from the laser itself or the opto-mechanical assembly. Forexample, U.S. Patent Application Publication No. US 20160025842 A1,entitled “System and Method for Monitoring Optical Subsystem Performancein Cloud LIDAR Systems” describes the benefits of laser output powermonitoring for a LIDAR system designed for cloud measurements. Usingoptical power monitoring in embodiments of the multi-wavelength LIDARsystem of the present teaching can improve upon these prior art systems,providing better angular resolution as well as other features ofmulti-wavelength LIDAR operation.

Power monitoring elements of the present teaching monitor lightreflected off of optical devices within the illuminator. The reflectedlight detected within the illuminator can also be used not just forpassive monitoring purposes, but also to provide additional control ofthe laser current bias. A laser diode has a range of operating biascurrents. In many laser systems, not necessarily those for the LIDARapplication, lasers are operated in closed loop fashion where thereceived photodiode current from the monitor diode serves as an input toa bias control loop. By monitoring and maintaining a constant value ofthe monitor photodiode current, which is a largely linear function ofthe incident power, the system will be able to react to changes insystem stability, such as temperature or mechanical shifts, and tomaintain an improved output power stability. Also, this monitoring andcontrol of the laser bias can accommodate some amount of degradation ofthe laser over its lifetime, without loss of optical power at the systemlevel.

Some embodiments of the performance monitor for LIDAR systems of thepresent teaching monitor for one or more parameters of the light. Thereflected light detected in the illuminator can be monitored for laserwavelength, optical power, pulse timing, and pulse frequency. Thewavelength can be detected by using a power monitor including a receiverthat is not simply a photodiode, but instead a more complicated set ofoptics that allows detection of wavelength as well as optical power. Ina LIDAR design where multiple wavelengths are used, particularly if thewavelengths are close in absolute value, it may be desired to monitortheir absolute or relative values in order to ensure that the systemparameters are as intended. Various methods of monitoring eitherabsolute wavelength of the laser, or the relative offset between lasersof different wavelength are known within the art. For example, anetalon-based device could be used as a wavelength monitor.

Multi-wavelength power monitoring also improves the system robustnessfor detecting whether a fault is caused by laser degradation or shiftsin optical performance. Multi-wavelength power monitoring also providesredundancy if one set of wavelengths should fail. A partial or fullfailure in operation of one set of wavelengths would still allow theability for partial operation of the system using the other set ofwavelengths if the optical monitoring for each wavelength isindependent.

Another feature of the multi-wavelength LIDAR systems of the presentteaching is that they can be resistant or immune to interference fromvarious optical sources. It is well known that some LIDAR systems can bejammed or fooled using relatively simple laser pointer, or laser rangefinder technology combined with a method to record and then replayemitted signals from the LIDAR system. It represents a potentiallysignificant risk to safety of the automobile and driver, if a hacker cancreate fake images of other cars, pedestrians, or even a wall.

Furthermore, as more LIDAR systems are deployed, it becomes increasinglyimportant to have systems that are immune to interference from otherLIDAR systems, whether from other vehicles or stationary trafficmonitoring. In various embodiments, encryption of the pulse signals maybe included. However, even without encryption, the multi-wavelengthsystems according to the present teaching can confirm that data receivedis genuine based on wavelength with or without the use of encryptedpulses. For example, some embodiments of the multi-wavelength LIDARconfigurations of the present teaching allow the independent use of eachwavelength, such that if one wavelength is “jammed”, the otherwavelength would still be available in order to continue operation.These systems can confirm a potential security risk, and/or enable acontrolled response to unintended system interference. Thus, themulti-wavelength LIDAR systems of the present teaching provide enhancedsecurity and prevent blinding, jamming, replay, relay, and spoofingattacks compared to a single-wavelength LIDAR.

Another feature of the multi-wavelength LIDAR systems of the presentteaching is improved system accuracy, and/or reduced requirements on thereceiver and processing electronics. These benefits are achieved byusing multiple wavelengths. Consider the sampling rate required for anautomotive LIDAR system. A car moving at 100 kilometers per hour (kph)is traveling at roughly 28 millimeters per millisecond (mm/msec). If twocars are approaching each other, then the relative distance willdecrease at twice that rate, or 56 mm/msec. For a system that isaccurate across the full field-of-view, with a distance accuracy of 50mm inches) for each measurement point, we need to be able to scan thecomplete FOV during that time.

For simplicity, assume a multi-source LIDAR system using 1,000 laserclusters, corresponding one-to-one with particular desired 3D pointwisemeasurement locations. For position accuracy across the full FOV, asdescribed above, we would need to scan through all 1,000 lasers every 1msec. For a single-wavelength system, where we can only operate anddetect one laser at a time, this means we have only 1 microsecond (usec)per laser to acquire the position information for that measurementpoint. In a multi-wavelength LIDAR system, where we can operate twolasers of different wavelength simultaneously, the time allowed perlaser will double and we can have 2 μsec per measurement point. Thisadditional time can be used to improve the performance and/orreliability in or more ways. For example, the additional time can beused to improve the signal integrity, for example, by taking additionalsamples and averaging. The additional time can also be used to reducethe data rate required for the electronics. In addition, the additionaltime can be used to implement a more robust coding scheme.

Another feature of the multi-wavelength LIDAR systems of the presentteaching is the use of higher-level modulation and coding of the laserpulses. This modulation and coding can take the form of numerous knownhigher-layer coding techniques for optical signals that are used toimprove photon-efficiency of the receiver, encrypt signals and/orincrease immunity to interference. By incorporating higher-ordermodulation instead of sending a simple binary, on/off, pulse, we canenable the implementation of coding/encryption. Coding/encryption isdesired because it allows reduction of security risk and interferencefrom other laser systems. Known higher-layer modulation and codingschemes that can be implemented include, for example, pulse positionmodulation, phase modulation, frequency modulation including sub-carrierfrequency modulation, amplitude modulation, polarization modulation,spread spectrum and others.

FIG. 10 illustrates an embodiment of a simple coding scheme 1000 formulti-wavelength LIDAR according to the present teaching. The top graph1002 presents a simple non-encoded binary pulse optical power as afunction of time. The pulse width is typically 30 to 200 nanoseconds(nsec) in length, since most LIDAR systems have an overall window perpulse of only 1 μsec.

The bottom graph 1004 illustrates a simple amplitude based codingscheme, where we have broken down the pulse into six sections, each withfour possible levels. The first section might be fixed at maximum power,and used for timing of the TOF measurement, as well as setting thereference signal amplitude for the remaining pulse. The last fivesections, however, could be varied in signal level with four possibleamplitude levels. The described pulse will then give us 1,024 uniquepulse shapes. Thus, each laser in a 1,000 laser LIDAR system can haveits own pulse shape or uniquely coded signature. Having each laser withits own uniquely coded signature allows the receiver electronics todecode the signal and identify exactly which laser fired that signal.Security is improved both through the uniquely coded signature of eachlaser, and by being able to close the loop between receiver andtransmitter in terms of matching an expected uniquely coded signaturefor any given laser pulse and compare to actual pulse received.

Another feature of the multi-wavelength LIDAR systems of the presentteaching is the use of parallel processing. Adding higher-ordermodulation in order to code/encrypt the laser pulses, requires higherspeed electronics and more complex signal processing methods. Theoptical-electrical (O/E) bandwidth of the complete laser circuit, madeup of the laser driver, laser, and physical interconnect must besufficiently high to accommodate the higher-order modulation bandwidth.

The receiver bandwidth can be a constraint. For better optical couplingor to realize more light captured by the receiver, the aperture (i.e.photosensitive area) of the receiver photodiode(s) should be large.However, for improved bandwidth and frequency response, it is desirableto have a smaller aperture size. As such, a compromise can be made.Using multiple-wavelengths can help by enabling parallel processing ofmeasurement points, which effectively creates additional time that canbe used to help reduce the system constraints for electronics,bandwidth, and receiver design. For example, referring to FIG. 10 , wehave depicted the optical pulse shapes with zero rise and fall times. Areal system will have finite rise and fall times for the optical signalthat, if long enough, would degrade the amplitude modulation. Someembodiments described herein enable overall optimization of theoptical-electrical RF performance of the receiver in support ofcoding/encryption.

In a single-wavelength LIDAR system, or a LIDAR system that does notrely on multiple independent wavelengths, only one laser is typicallybiased at a time because the receiver cannot distinguish between lightfrom multiple lasers of the same wavelength. It is known, for example,in multi-wavelength communications applications, that systems with morethan one wavelength enable simultaneous operation of more than onelaser, so-called parallel operation. Similarly, LIDAR systems that usedifferent wavelengths can enable coding/encryption of the pulses at alower data rate. This results in allowing the use of lower-speed andmore inexpensive components, and/or enabling more powerful encryption.This is in part because the receiver can be designed to simultaneouslymeasure the light with different wavelengths. The receiver for amulti-wavelength system might, for instance, might have two separatephotodiode elements each filtered to receive only the light from onewavelength.

Early LIDAR systems for automotive applications, including the firstself-driving cars, employ a single high-power laser that is reflectedoff a spinning mirror, where the reflected laser beam is used to performscanning of the environment. LIDAR systems using a spinning mirror haveadded complexity and, therefore, there are potential mechanicalreliability issues with the moving parts. To avoid the use of a spinningmirror, LIDAR systems have been proposed that replace a single laser,with a plurality of lasers. See, for example, U.S. Pat. No. 5,552,893entitled “Distance Measuring Apparatus”. By using a plurality of lasers,it is no longer necessary to utilize a spinning mirror to perform thescanning. The plurality of lasers can be shifted as a group in somefashion, mechanically or optically, or the number of individual laserscan be sufficient to have a one-to-one relationship with the desiredgranularity of the pointwise mapping of the environment. In general, itis highly desired to have a “solid state” system with no moving partsfor better mechanical reliability, but there is a significant cost andcomplexity associated with having a plurality of lasers large enough tohave a one-to-one mapping.

In some aspects of the LIDAR systems of the present teaching, VerticalCavity Surface Emitting Laser (VCSEL) arrays are used to provide aplurality of lasers. VCSEL arrays have many desirable features, such asproviding a circular beam divergence, being capable of beingmanufactured on a single substrate, high electro-optic efficiency, andlow cost.

As described earlier, some estimates indicate that to provide adequatetime to avoid a collision, a LIDAR system should be able to resolve apedestrian walking in front of the car at a distance of 30 meters. Thepedestrian cross section would roughly be 280 mm, so the angularresolution required for the LIDAR system needs to be ˜0.5 degrees. For aplurality of lasers to have a one-to-one mapping between the laser andthe 3D pointwise mapping of the environment, the number of requiredlasers is calculated by dividing the system FOV by the angularresolution requirement. A typical LIDAR system might have a 20-degreeFOV, which may include 40 lasers to meet the angular resolutionrequirement to resolve a pedestrian at 30 meters. Note that thiscalculation is for only one direction, which would generate 2Dinformation. A similar calculation is required for the perpendiculardirection. If the system had a 20-degree FOV, both horizontal andvertical, 1,600 lasers would be required. A system using 1,600 separatelasers presents significant challenges for complexity of operation,assembly, and cost. Known LIDAR systems disclose using a smaller numberof lasers, but shifting them as a group to achieve the requiredresolution using various devices, such as voice coils, piezoelectrictransducers, stepper motors, shape memory translators, and vibratorymotors. See, for example, U.S. Pat. No. 7,544,945 entitled “VerticalCavity Surface Emitting Laser (VCSEL) Array Laser Scanner”

Some known LIDAR systems use an array of VCSELs together with a moveablelens system to shift the beams. As the lens is moved, each laser beam ispointed in a different direction. Full mapping of the field-of-view isachieved by selectively electrically biasing each laser, combined withappropriate movement of the lens system. VCSEL arrays can be formed froma plurality of individual VCSEL chips, or sub-arrays of VCSEL chips. Themovable lens can be actuated by a piezoelectric transducer or othermechanical means. See, for example, U.S. Pat. No. 6,680,788 entitled“Scanning Apparatus and Associated Methods”

Some embodiments of LIDAR systems according to the present teaching usea plurality of semiconductor lasers with lasers having more than onelaser wavelength. Specific embodiments described herein include usingsemiconductor lasers of different wavelengths to enable a LIDAR systemwith relatively higher granularity for 3D pointwise mapping as comparedto single laser wavelength system. One feature of the present teachingis that the relatively higher granularity for 3D pointwise mapping canbe achieved without requiring the use of a movable lens system or othermovable parts within the LIDAR system.

Embodiments of LIDAR systems according to the present teaching that usemultiple laser wavelengths can significantly improve upon thegranularity, or angular resolution, as compared to a system using onlyone laser wavelength, while maintaining a compact mechanical size forthe overall system. As described previously, it is highly desired thatan automotive LIDAR system have a granularity of less than 0.5 degreesfor each measurement point across the FOV in order to be able to detecta pedestrian at 30 m with high accuracy. A perfectly collimated lightbeam is one that has zero degrees of divergence, but in a real system,it is understood that the properties of the emitted laser light beam, aswell as deviations from ideal in the optics and mechanical assembly,prevent the realization of a perfectly collimated light beam.

There are two general methods for collimating lasers beams in LIDARsystems that have a plurality of lasers. In the first method, the lightfrom each individual laser is collimated separately, by its owndedicated lens or lens system. In the second method, collimation ofmultiple laser beams is performed with a shared optical system.

Another prior art LIDAR system comprises lasers located on a curvedsubstrate and uses individual lenses for each laser. See, for example,U.S. Patent Publication No. 2015/0219764 entitled “Low Cost Small SizeLIDAR for Automotive.” This patent application describes a system withan FOV and collimation that can be independently set. That is, thedegree of collimation can be controlled by the individual set of opticsused for each laser, independent of the desired FOV. However, there arephysical limits to the collimation. A VCSEL device has both a finitebeam diameter, as well as a finite divergence angle. For a typical 850nm VCSEL device, with a numerical aperture (NA) of 0.2, a refractiveoptic with a focal length (and physical diameter) of several mm would berequired, to produce a beam with less than 0.5 degree divergence. Thediameter of the individual lenses will physically limit the spacingbetween adjacent lasers. It should be noted that the individual lenssystems used for collimation of the light from the plurality of lasers,might be in the form of an array, rather than physically separateelements. However, the limits on the spacing of the adjacent lasers willstill be a factor.

Some prior art LIDAR systems use a plurality of VCSELs arranged in apattern on a substrate combined with a one-to-one plurality ofmicro-lenses formed over each VCSEL device. See, for example, U.S.Patent Publication No. US2015/0340841 A1 entitled “Laser Arrays forVariable Optical Properties.” Each lens will both collimate, and set theprojection angle for the laser beam from the VCSEL device. A pointingangle can be achieved by offsetting the primary lens axis from the VCSELcenter axis. The VCSELs in some known LIDAR collimation systems areback-side illuminating and the lenses are on the opposite surface of thesubstrate from the VCSEL apertures. In these VCSELs, the substrate needsto be transparent to the wavelength of the VCSELs. It is also possibleto use top-side illuminating VCSELs and form the lenses on the samesurface as the VCSEL apertures.

Some prior art LIDAR systems using multi-emitter VCSELS have a compactdesign with the lenses and spacing of the VCSEL elements on the order ofa few hundred microns. These systems provide an optical systemessentially on the same mechanical scale as the VCSEL array chip.However, these systems suffer from significant drawbacks, including aninability to achieve a small angular resolution because the divergencefrom an individual device cannot be arbitrarily low. Such a system willbe constrained in performance by the size and curvature of the lenses.In such a system, divergence cannot be set arbitrarily small,particularly for the devices where the lens and emitting VCSEL apertureare offset. The beam diameter and the divergence are linearly related.

By inspection, we know the magnification of this system is small, andthat the laser beam diameter cannot be significantly expanded by such alens system. For this approach to have divergence of less than 0.5degree, the numerical aperture of the VCSEL device itself must start outto be small initially, much lower than typical 0.2 NA for an 850 nmtop-side illuminated VCSEL. Requiring an inherently small numericalaperture for the VCSEL device may limit the practicality of thisapproach by impacting the VCSEL output power and wavelength. Also, sincethe same lens is used for collimation and for setting the projectionangle, this places an additional constraint on the lens design as theradius of the lens becomes dependent on the desired projection angle,which is likely not the same lens radius as the optimum radius forcollimation.

The second method of collimation found in prior art LIDAR systems thatinclude a plurality of lasers at a single wavelength is to use a sharedoptical system to collimate multiple lasers. In some of these systems, asingle large lens is used to both collimate as well as set theprojection angle of each VCSEL device. It should be noted that insteadof a single lens, two or more lenses could be used as well, withoutchanging the underlying concept of a shared lens system for collimation.One aspect of using a shared optic for both collimation and projectionangle is that there is a direct mapping between the lateral position ofthe VCSEL device relative to the central axis of the lens and thepointing angle of the projected laser beam. The lateral distance betweentwo VCSEL lasers of the same, or similar, wavelength will correspond tothe difference in projection angles created by the shared lens system.

Furthermore, since the VCSEL device is not an ideal point source, butinstead has a finite lateral size, there will be an additionaldivergence that cannot be reduced by the optics without also shrinkingthe FOV of the overall optic system. Also, the shared-optic approachusing lasers with the same or similar wavelength may lead to beamoverlap or gaps in the 3D measurement span depending on the finite sizeof the VCSEL, the divergence of the collimated beams, the number ofVCSEL devices, and the FOV, among other parameters.

One feature of LIDAR systems of present teaching is the use of VCSELchips with clusters of emitting apertures to take advantage of thehigher optical power and large diameter cluster provided by thesedevices. As described herein, a VCSEL device is not an ideal pointsource, but rather has a finite lateral dimension. Furthermore,high-power top-emitting VCSEL lasers used for LIDAR illuminationtypically use multiple light emitting apertures to reach the requiredhigh-power output. These multiple apertures form a cluster or group, andideally are located as close as physically possible, while stillmaintaining the required electro-optic efficiency.

FIG. 11 illustrates a drawing of a prior art cluster VCSEL device 1100that comprises a cluster 1102 formed by twenty-one individual apertures1104. The twenty-one individual light emitting apertures 1104 form acluster 1102. The VCSEL apertures 1104 within the VCSEL cluster 1102 areelectrically connected in parallel and so must be biased together. Therequired output power from the cluster VCSEL device 1100 differs foreach system design, but generally exceeds 1 W of peak optical powerduring pulsed operation, in order for the overall LIDAR system to meetacceptable signal-to-noise ratio. See, for example, U.S. Pat. No.8,247,252 entitled “High Power Top Emitting Vertical Cavity SurfaceEmitting Laser.”

Some critical parameters in the design of the optical system are thelateral size (dimension) of a cluster VCSEL chip, the diameter of theVCSEL cluster 1102 combined with the numerical aperture of theindividual VCSEL apertures 1104. These parameters will determine thelens requirements for collimation. The larger the lateral dimension ofthe VCSEL cluster 1102, the larger the focal length required forcollimation. Larger lateral dimensions generally lead to a physicallylarger optics system.

One feature of using VCSEL devices in LIDAR systems according to thepresent teaching is the ability to have multiple cluster VCSEL deviceson a single chip. FIG. 12A illustrates an array 1200 comprising multiplecluster VCSEL devices 1202. FIG. 12A illustrates a twenty-five clusterVCSEL devices 1202 in a two-dimensional array. The array is formed withcontacts 1204 for twenty-five individual cluster VCSEL devices 1202 thatcan be individually biased.

FIG. 12B illustrates an array 1250 comprising multiple cluster VCSELdevices 1252. FIG. 12B illustrates that the array is arrange to includefive cluster VCSEL devices 1252 connected with contacts 1254 that formfive bars with each bar including five cluster VCSEL devices 1252. Itwill be evident to those familiar with the art that a single monolithic2D VCSEL array can be produced as well.

FIG. 12C illustrates a top-view of an anode metal contact pad 1270 of achip 1272 comprising multiple cluster VCSEL devices 1274 in a 2Dmonolithic VCSEL array. The chip illustrated in FIG. 12C is a top-sideilluminating VCSEL array. All the anodes of all VCSEL in a single columnare connected together with a single metal contact.

FIG. 12D illustrates a bottom-view of a cathode metal contact pad 1276of the chip 1272 comprising the multiple cluster VCSEL devicesillustrated in FIG. 12C. All the cathodes in a single row are connectedtogether with a single metal contact. With this pattern ofmetallization, individual VCSEL devices 1274 (FIG. 12C) can be operatedby biasing each row and column contact at the desired bias level. Forthis particular embodiment with 5 rows and 5 columns, only 10 electricalconnections are required versus 25 electrical connections if the VCSELdevices 1274 were individually connected. One skilled in the art willappreciate that this is one of numerous possible electrical addressingconfigurations and that the present teaching is not limited toparticular row and column geometries for the emitters. This advantage inreducing the number of electrical connections is greater as the size ofthe 2D VCSEL array increases. In general, when the anodes of one groupof laser emitters is connected to one contact, and the cathodes of asecond group of laser emitters is connected to a second contact, onlythose individual lasers belonging to both the first and second group oflaser emitters, i.e. those that have an anode and a cathode connected,will be energized when the first and second contacts are appropriatelybiased. The use of one contact connected to anodes of one group of laseremitters and a second contact connected to cathodes of a second group oflaser emitters can be used to energize one laser emitter, or groups oflaser emitters, for a particular bias condition, depending on theconfiguration of the connections.

Prior art LIDAR systems do not utilize different laser wavelengths toenable improvements to the angular resolution of the LIDAR system. Onefeature of the LIDAR systems of the present teaching is that they usemultiple laser wavelengths to enable finer angular resolution andperformance in a low-cost, compact optical design. Furthermore,multi-wavelength LIDAR systems of the present teaching provide a simplepath to improved security and parallelization as described herein.

One feature of the LIDAR systems of the present teaching is the use ofmultiple wavelength laser sources for a LIDAR system that uses a sharedlens system to both collimate and project the laser beams over a desiredfield-of-view. FIG. 13 illustrates an embodiment of a multi-elementemitter laser source 1300 for a multi-wavelength LIDAR system of thepresent teaching. A plurality of lasers including cluster VCSEL devices1302 are all located on a common surface 1304. The surface may be flat,as shown, or curved.

FIG. 13 illustrates a multi-element emitter laser source 1300 with twodifferent VCSEL wavelengths interleaved uniformly in the verticaldirection. The embodiment shown in FIG. 13 illustrates a single commonsubstrate 1304, but it will be clear to those skilled in the art thatmultiple substrates could also be used. There are six VCSEL bars 1306,1308. The cluster VCSEL devices 1302 of the bars 1306 emit at one commonwavelength. These are the bars 1306 labeled “VCSEL λ1” in the figure.The cluster VCSEL devices 1302 of the dark bars 1308 emit at a differentwavelength. The bars 1308 are labeled “VCSEL λ2” in the figure. A totalof thirty cluster VCSEL devices 1302 are shown.

The illuminator used in connection with the multi-element emitter lasersource 1300 of FIG. 13 uses a shared lens system for both collimationand projection of the beams over the desired FOV. FIG. 14 illustrates adiagram of a cross-section of an embodiment of an illuminator 1400 for amulti-wavelength LIDAR system of the present teaching. The illuminator1400 includes the multi-emitter laser source 1402, and a projectionelement 1404 comprising a wavelength multiplexer 1406 and a lens 1408.The projection element 1404 is used to project the laser beams 1410,1412 emitted from the laser source 1402. The emitters for themulti-emitter laser source 1402 are located on a VCSEL substrate 1414.FIG. 14 illustrates how the light from two VCSEL bars, of differentwavelength, travels through the system. For clarity, only the laserbeams 1410, 1412 from two VCSEL emitters are shown ray traced.

In FIG. 14 , the projection element 1404 comprises two optical devices1406, 1408. The first optical device is a wavelength multiplexer 1406that is a wavelength sensitive optic that acts to combine the laser beam1410 at one of the two wavelengths and from one optical path with thelaser beam 1412 at the other of the two wavelengths that is on anotheroptical path onto a common optical path. In some embodiments, wavelengthmultiplexers comprise a diffractive optic that is designed tosubstantially shift the optical path of one wavelength while letting thesecond wavelength pass through undisturbed. Diffractive optics elementsare well known in the art and can be used to provide precise beamsteering and beam shaping of lasers. In addition, diffractive opticalelements can be wavelength selective. In other embodiments, an array ofrefractive optics, such as prisms, is used. The second device is a lens1408 that is used to further project and shape the laser beams 1410,1412 to form a desired pattern of beam shapes and beam positions at thetarget plane of the LIDAR system.

Light from the beam profiles formed at a target plane by the illuminatoris reflected from the surface of objects in that target plane. A targetplane in a LIDAR system is a virtual reference point that operates overa complete range and FOV. There are many different target planes atvarious distances from the LIDAR module such that the system cangenerate three-dimensional representation of the objects in thefield-of-view and range being probed by the LIDAR system.

A portion of the light reflected of the surfaces of objects illuminatedby the optical beam profiles in the target plane is directed toreceivers. The receivers detect the light, converting the receivedoptical signal to an electrical signal. A controller, connected to thelight sources and to the receiver, converts the received signal into ameasurement point cloud. The angular resolution of points in themeasurement point cloud depends of the relative position of the beamprofiles at a target plane, as described further below. It will be clearto those skilled in the art that many other variations of the embodimentof the illuminator 1400 illustrated in FIG. 14 exist. For example, VCSELlasers may be located in a common surface, either planar or curved. Itwill also be clear that some deviation of the VCSEL away from a centralsurface, curved or flat, is allowed without changing the principles ofthe embodiment of FIG. 14 .

FIG. 15A illustrates a measurement point cloud 1500 for an embodiment ofa single-wavelength 2D laser source illumination of the presentteaching. The distance represented between the vertical spacing 1502 ofthe measurement points 1504 determines the vertical angular resolutionand horizontal spacing 1506 of the points on the point cloud determinesthe horizontal angular resolution of the point cloud.

FIG. 15B illustrates a measurement point cloud 1550 for an embodiment ofa two-wavelength 2D laser source illumination of the present teaching. Ameasurement point corresponding to a VCSEL with λ1 is shown as a circle1552, a measurement point with a VCSEL with λ2 is shown as a triangle1554. It is useful to think of this measurement point cloud as acomposite point cloud that includes a point cloud derived fromreflections received at λ1 and a point cloud derived from reflectionsreceived at λ2.

The measurement point cloud 1550 illustrated in FIG. 15B can be realizedusing the multi-emitter laser source with the pattern of VCSEL emittersof different wavelengths illustrated in FIG. 13 together with theilluminator configuration of FIG. 14 . Part of the light from opticalbeams generated by the illuminator at the target plane is reflected bythe surface of an object and incident on one or more optical receiversthat are capable of detecting light at particular wavelengths. Theresulting measurement point cloud 1550 includes points representinglight from the different beam profiles that are at differentwavelengths.

Referring to FIGS. 13-15 , different wavelength VCSEL bars 1306, 1308occupy different rows of the laser source in the vertical direction, andindividual VCSEL devices 1302 in different rows have their centersoffset in the horizontal direction. The optical beams from emitters indifferent wavelength bars are projected by the projection element 1404so that the optical beam positions are slightly offset at the targetplane in the vertical direction. This causes the offset 1556 in themeasurement point cloud. The offset in the center position of the VCSELsin adjacent bars, together with the design of the projection elementcauses the measurement points representing each wavelength to beinterleaved horizontally along the offset vertical lines. The angularresolution of the measurements in a given dimension is directly relatedto the offset of the points in that dimension, which is related directlyto the positions of the optical beams in that dimension at the targetplane.

Referring to both FIG. 15A and FIG. 15B, a performance tradeoffassociated with using a two-wavelength solution is clear. In theembodiment of FIG. 15B, the optical beams at one wavelength travelsubstantially uninterrupted, but the optical beams at the secondwavelength are intentionally shifted in position to substantiallyoverlap in one direction with the optical beams at the first wavelength.The offset 1556 in position of the optical beams at each wavelengthindicated in the drawing in FIG. 15B can be adjusted based on the designof the wavelength multiplexer. For example, in some embodiments, thisrequires appropriate design of the wavelength multiplexer 1406 in FIG.14 . In various embodiments, various devices in the projection elementare used to position the beams of the lasers at the two wavelengths.These same devices, or other devices, may alter the beam shapes as welltheir positions at the target plane.

As compared to the single wavelength embodiment of FIG. 15A, theembodiment of FIG. 15B doubles the angular resolution in a preferreddirection, in this case the horizontal direction, at the expense ofhalving the angular resolution in the perpendicular direction, whilekeeping the overall physical size of the system relatively constant. Insome applications, finer resolution in one direction may be preferred ornecessary, for instance if instead of a pedestrian, the system needs todistinguish a pole or tree with a cross-section of only 100 mm. At 30 m,we would need an angular resolution less than 0.15 degrees. Poles andtrees are tall but narrow, and so it could be highly desired to have avery small angular resolution in the horizontal direction, at theexpense of wider angular resolution in the vertical. In someembodiments, the angular resolution of the measurement point cloud isless than 0.4 degree at a predetermined distance from the target planeto the optical projection element.

FIG. 16 illustrates a measurement point cloud 1600 for an embodiment ofthe two-wavelength LIDAR of the present teaching. In this embodiment, wehave not given up any resolution in the vertical direction, but havebeen able to add a single line 1602 with double the angular resolution.At this one vertical position of line 1602 in the FOV, we have doublethe angular resolution horizontally. It should be also noted that, atthe central position, we have redundancy in the event one set of VCSELwavelengths fail to operate. Multiple lines like single line 1602 can beadded at the expense of complexity of the multiplexer.

FIG. 17 illustrates a measurement point cloud 1700 for an embodiment ofa three-wavelength LIDAR of the present teaching. In this embodiment,the measurement point cloud is generated by adjusting the beam positionat a target plane for three wavelengths. The field-of-view andresolution provided by this measurement point cloud is dependent on thedetails of the beam profile provided by the laser source and projectionelement. In some embodiments, the profile is provided by collimating thelaser emitters using a lenslet. In some embodiments, the lenslet isintegrated with the VCSEL device. The collimated beams at the multiplewavelengths are positioned and directed to wavelength sensitive elementthat projects the beams at different wavelengths along a commondirection toward the target. With proper design of the wavelengthsensitive element, and physical layout of the VCSEL, we can generate asystem with 3× the angular resolution in one direction, at the expenseagain of about ⅓ resolution in the perpendicular direction asillustrated in the measurement point cloud 1700. Because light from eachof the three wavelengths is independently received, the positionrepresented by each measurement point can partly, or completely,overlap, increasing the resolution of the measurement point-cloud and/orproviding redundant measurement points. In the embodiment of FIG. 17 ,the increased resolution of the measurement point cloud 1700 is providedin a horizontal direction. In various embodiments, the direction ofhigher angular resolution can also be changed, becoming vertical or evendiagonal depending on layout of the VCSEL devices and specific design ofthe wavelength multiplexer.

One feature of the present teaching is that the light sources that areeither single element emitters or multi-element emitters operating atdifferent wavelengths do not need to be located on the same surface, andthe surfaces may be oriented along different spatial planes inthree-dimensional space. For example, the planes may be on twoorthogonal planes. In some embodiments, we use a plurality of surfaceemitting lasers made up of at least two groups of lasers with differentwavelengths. We also make use of three-dimensional space and each groupof lasers are oriented in two or more surfaces, planar or curved, thatare not necessarily orthogonal. In this embodiment, the packaging andoptical alignment complexity increases relative to embodiments in whichthe lasers are co-located on a common surface, but we are able toincrease the resolution angle across the full field-of-view in bothorthogonal directions, without any compromise. This provides both higherprecision as well as full access to all the capabilities associated withmore than one wavelength. That is, it is possible to realizesimultaneous operation, redundancy, security and other features ofmulti-wavelength operation.

FIG. 18 illustrates an embodiment of an illuminator 1800 for amulti-wavelength LIDAR system of the present teaching. In thisembodiment, we use a plurality of surface emitting lasers made up of atleast two groups of lasers with different wavelengths, VCSEL λ1 1802,and VCSEL λ2 1804. We also make use of three-dimensional space and VCSELλ1 1802, and VCSEL λ2 1804 are oriented in two surfaces that areorthogonal. The beams are combined by use of a wavelength multiplexer1806 that passes one wavelength, while reflecting the second wavelength.

FIG. 18 illustrates an essential principle of combing the light from twosets of lasers VCSEL λ1 1802, and VCSEL λ2 1804, with two differentwavelengths. One set of emitters of one wavelength, VCSEL λ1 1802, is ona common surface. The second set of emitters of a different wavelength,VCSEL λ2 1804, is on a second surface, oriented orthogonally. Thewavelength multiplexer 1806 can be realized, for example, by use of athin film filter that allows the first wavelength to pass throughundeflected, while the second wavelength is deflected at 45 degrees, andthe output beams are combined. For simplicity, we have shown themultiplexer 1806 in the shape of cube formed by two equal prisms oftriangular cross-section, where the thin film filter that reflects orpasses the wavelengths is located at the central plane of the cube,where the two triangular prisms touch.

The positions of the two substrates of VCSEL λ1 1802, and VCSEL λ2 1804can be shifted laterally, relative to the wavelength multiplexer 1806 tocreate the desired overlap or interleaving of the two beams. FIG. 19illustrates a measurement point cloud 1900 that can be generated withthe illuminator embodiment of FIG. 18 .

One feature of the present teaching is providing a multi-wavelengthLIDAR system where each wavelength has a different angular resolution.The beam profiles of the different wavelengths can be substantiallydifferent in order to allow for different operating ranges. For theseembodiments, we make use of the fact that we have a receiver that canseparately detect multiple-wavelengths.

FIG. 20 illustrates an embodiment of a measurement point cloud 2000generated using two multi-element emitter sources in a multi-wavelengthLIDAR configuration of the present teaching. As can be seen from thepoints 2002 of the measurement point cloud 2000 generated from a firstwavelength, and the points 2004 generated from the second wavelength,each wavelength has a different angular resolution in the vertical axis.Thus, the vertical resolution of the measurement point cloud for the twowavelengths is substantially different, in this case by a factor of two.

In the embodiment of FIG. 20 , there is also a region of overlap of thetwo wavelengths at a line 2006, which results in a higher angularresolution at that location. Such a design enables lower cost andcomplexity because it relies on fewer VCSEL lasers to realize aparticular resolution as compared to a single wavelength design. Forsome particular applications, a sparser 3D measurement may be acceptablein the vertical FOV. One such example, is when the vertical height istaller than the vehicle. In the embodiment shown in FIG. the change inangular resolution is generated from an asymmetric layout of the VCSELdevices on their substrate(s).

FIG. 21 illustrates a measurement point cloud 2100 generated by anembodiment of a multi-wavelength LIDAR where the density of measurementpoints 2102 from one wavelength across the full field-of-view is halfthat of the measurement points 2104 of a second wavelength according tothe present teaching. This configuration is desirable if the secondwavelength is used in a different manner then the first wavelength. Forexample, if the two wavelengths are intended to be used for differentdistance ranges, then the angular resolution can be different. This isshown in the table from FIG. 4 . Other reasons for a difference inangular resolution, might include using the second wavelength for afaster overall scan, or to improved security, or for simple partialredundancy.

One feature of LIDAR systems of the present teaching is that additionalwavelengths can be easily added. With the use of additional wavelengths,and a more complex wavelength multiplexer, more measurement points canbe added. FIG. 22 illustrates an embodiment of a measurement point cloud2200 of the present teaching. The point cloud represents measurementpoints provided by four different wavelengths. The points associatedwith each wavelength are illustrated as different shapes in the 3Dmeasurement point cloud.

One feature of the present teaching is that the optical system can beconfigured such that the generated 3D point cloud has different angularresolution along different directions. The optical configurationincluding the spacing of the elements in a VCSEL array together, thecollimation, and the combining devices in the projection element arearranged to change the angular resolution in a given direction thatmaximizes the density where desired and minimizes the density where notrequired. In some embodiments, the VCSEL devices are laid out in aregular uniform pattern, where the density is substantially constant inthe horizontal and vertical direction, but not necessarily required tobe the same in each direction. Such a configuration can assist in themanufacturing process since it is desirable to have a relatively uniformlayout to ease the assembly and electrical connection of the VCSELdevices to a package.

FIG. 23A illustrates a VCSEL array layout 2300 for an embodiment of amulti-wavelength LIDAR where the angular resolution changes in aparticular direction according to the present teaching. The substratecomprises twenty-five VCSEL lasers 2302, laid out in a regular patternwhere the spacing is constant in each direction, but different betweenthe two directions. In the horizontal direction, each VCSEL device 2302is offset with a uniform spacing of x 2304 between devices, and in thevertical with a uniform spacing of y 2306 between devices. Thus, theVCSEL laser array has a uniform spacing in the x-direction and a uniformspacing the y-direction, but the two spacings are not the same.

FIG. 23B illustrates a measurement point cloud 2350 generated by anembodiment of a multi-wavelength LIDAR with varying angular resolutionaccording to the present teaching. A non-uniform 3D point cloud that canbe created by the optical system from the uniform VCSEL structure ofFIG. 23A. In the embodiment of FIG. 23B, the spacing of the projectedmeasurement points varies both in the horizontal and in the verticaldirections and is no longer uniform. In the vertical direction, thespacing for the bottom two rows of devices is u 2352, and with each rowincreases by an additional spacing u 2352, to become 4u 2354 at the top.In the horizontal direction, there is a symmetric pattern centered aboutthe central axis, with spacing, p 2556, for the two adjacent columns andthen increasing to a spacing, 2p 2358. One feature of this embodiment isto have a lower number of VCSEL overall, for a given field-of-view. Sucha configuration has relatively low cost and is useful for applicationswhere high angular accuracy is only needed in a portion of the desiredFOV. One skilled in the art will appreciate that numerous other patternscan be generated by LIDAR systems according to the present teaching.

Some embodiments of the LIDAR systems of the present teaching have amodular design where the VCSEL lasers are placed in a separate assemblythat is designed to be used with one or more different optical designs,with different FOV. In these embodiments, only a single VCSEL laserassembly needs to be manufactured to address the needs of multipleautomobiles and/or applications. We can understand that differentvehicles, with different target uses and costs, will have widely varyingrequirements for number of LIDAR systems, and the system range and FOV.Thus, one aspect of the LIDAR systems of the present teaching isconstructing a modular LIDAR system. In these modular systems, thecommunication and data transfer to the modular VCSEL assembly may bedigital, and all analog processing may occur within the modular unit.

It should be understood that varying angular resolution in differentdirections according to the present teaching as described in connectionwith FIGS. 23A and 23B is independent of number of wavelengths used.Even a single wavelength LIDAR system can benefit from varying theresolution in different directions.

The projection element of the illuminator can be constructed withvarious combinations of optical devices to achieve the desired beamprofile and positions at a target plane. In the multi-wavelength LIDARsystems according to the present teaching, one optical device is used asa wavelength multiplexer to combine the different wavelengths. Awavelength multiplexer allows the beam paths, or beam directions to bemodified as a function of wavelength. A variety of known wavelengthmultiplexer devices can be used. For example, thin film filters can beused to pass one wavelength, while reflecting other wavelengths. Also,diffractive optics can be used. If the lasers used are linearlypolarized, then polarization sensitive optics can also be used. For twowavelengths LIDAR systems where each wavelength is linearly polarized(with one polarization orthogonal to the other), the wavelengthmultiplexer can be a polarizing beam splitter. For LIDAR systems withadditional wavelengths, manipulating the polarization state using waveplates, and reflecting or passing light based on the polarization, cancreate a suitable wavelength multiplexer using a polarizing beamsplitter.

In some particular embodiments, a region of space is sufficient tocombine the beams. In the case of multi-mode VCSELs, the beamcombination may be performed in free space without a common multiplexer.This region of free-space used to combine beams is referred to as afree-space optical element. FIGS. 24A-C illustrate such a free spacecombiner. FIG. 24A illustrates an embodiment of a multi-modemulti-emitter VCSEL laser source illuminator 2400 of the presentteaching. Two VCSEL arrays 2402, 2404 with five laser emitters 2406 eachare shown. The arrays 2402, 2404 are placed nominally in a single plane,although this is not required. The arrays can be offset or at an angleto each other. A separate lens system 2408, 2410 is used with each VCSELarray 2402, 2404 to set the projection angles. In practice, the arrays2402, 2404 are separated by only about 10 mm.

In operation, the beams from the two arrays become fully interleaved asshown in FIG. 24B within a very short distance (<100 mm). FIG. 24Billustrates a cross section of a projected beam profile 2430 of themulti-mode multi-emitter VCSEL laser source of FIG. 24A. Light beams2432, 2434 from the arrays 2402, 2404 are fully interleaved. With properpositioning of the lenses 2408, 2410, the angle of projection from eacharray can be adjusted to produce a uniform spacing. FIG. 24C illustratesthe projection angles for the multi-mode multi-emitter VCSEL lasersource of FIG. 24A. The beam projection angles 2450 shown include onlythe max 2452, median 2454, and minimum 2456 beam projection angles. Thecorresponding lasers from each array are spaced 10 mm apart, and thelaser beams are shown projected out to a distance of 1 meter.

Thus, a region of free space may be used within the projection elementto produce a desired profile of laser beams at the target planes in thefield-of-view and range of the LIDAR system. It will be clear to thosefamiliar with the art that the multi-emitter laser beams of differentwavelengths projected using free space can be aligned in such a way toproduce 3D measurement point cloud of adjacent points (for example asshown in FIG. 15B), or the measurement points could be made overlapping,partially or completely, as desired.

Another feature of the LIDAR systems of present teaching is theincorporation of a temperature monitor in close proximity to the VCSELdevices. Lasers are sensitive to temperature. The reliability of thelaser devices can be improved by controlling the bias to the laser as afunction of temperature. It is well known, for example, that the biasrequired for a certain fixed value of optical power will be lower at lowtemperatures. Some LIDAR systems according to the present teachingincorporate a temperature monitor in the transmitter to provide feedbackfor improved laser bias algorithms, laser lifetime reliabilityimprovement, and overall system performance monitor.

FIG. 25 illustrates an embodiment of a VCSEL array 2500 comprising adevice used to measure temperature located in close proximity to theVCSEL devices according to the present teaching. Two temperature sensors2502, 2504 are located on a single common substrate 2506. In variousembodiments of the LIDAR system of the present teaching, one or moretemperature sensors can be used. One temperature sensor may be adequateif the temperature gradient across the substrate 2506 is small, and orwell predicted. For example, the temperature sensor can be a thermistor.Thermistors are well known in the art to have a resistance that isdependent on temperature. By passing a signal through the thermistor,and measuring the current/voltage, the operating resistance and thustemperature of the thermistor can be calculated.

It is desirable for some LIDAR systems according to the present teachingto incorporate and calibrate temperature monitors for variousperformance measures during manufacturing. By calibration, we mean thecharacterization of the laser bias, temperature, and output power of thedevice, and the subsequent tuning of laser bias and output power as afunction of temperature to meet the required performance specificationswith suitable margins. Often this process is performed duringmanufacturing. The performance parameters, such as the laser bias andoptical power, obtained during the calibration process, can also bestored as a function of temperature as a reference in the system memory.

During operation of the LIDAR systems, the actual temperature can bemonitored and used in conjunction with the values stored in memory as alook-up table to set the laser bias. Alternatively, in combination withan optical power monitor, the actual values of output power, laser bias,and temperature during operation can be compared to the reference valuesto identify any significant change or degradation in the system, whichcan indicate of potential reliability issues. In variousimplementations, a LIDAR system detecting such changes could thencommunicate to the overall monitoring system for the automotive vehiclethat there is a need for potential service or repair.

The temperature sensor(s) 2502, 2504 can also be used to detect when theVCSEL laser devices 2508 are out of their intended operating range.Since automobiles are operated outside, potentially in extreme weatherconditions, it could be important to the reliability and safety of thesystem to know with some accuracy the temperature of the VCSEL laserdevices 2508. A system able to detect when temperature is outside ofoperation, could then take some action, such as preventing operationuntil ambient conditions meet operation criteria.

FIG. 26 illustrates an embodiment of a VCSEL array 2600 comprising anactive thermal control device 2602, 2604 used to control the VCSEL array2600 temperature according to the present teaching. Active thermalcontrol device 2602, 2604 are used to adjust the VCSEL temperature andbring it to within an operating range that is acceptable. Incorporationof active thermal control is sometimes desirable as it can be used toprovide a wider range of operating temperature of the LIDAR system. Thelocalized active thermal control device 2602, 2604 is co-located on thesubstrate(s) 2606 of the VCSEL array 2600 and constructed in such afashion to attain the desired thermal efficiency. The thermal controldevice 2602, 2604 can be a heating resistor element. However, a systemusing only a heating resistor element, would be able to heat but notcool the VCSEL. Alternatively, a thermal electric cooler (TEC) could beused as the active thermal control device that can both heat as well ascool.

The VCSEL substrate 2606 includes several thermistors 2608, 2610 as wellas thermal control devices 2602, 2604, which may be heating resistors.The thermistors 2608, 2610 are thermally coupled with the laser throughthe substrate and are in relatively close proximity to the lasers inorder to monitor the temperature. The system will pass current asnecessary through the heating resistors to bring the temperaturemeasured by the thermistors into the desired temperature range.

FIG. 27 illustrates a cross section of temperature-controlled VCSELarray 2700 comprising a thermo-electric cooler (TEC) 2702 for heatingand cooling the VCSELs 2704 according to the present teaching. Inaddition to having a thermistor 2706 in close proximity to the VCSELs2704, a thermistor 2708 may also be placed at the base of the TEC 2702in order to measure the base temperature.

Another feature of the LIDAR systems of the present teaching is the useof a highly integrated laser driver and VCSEL assembly where the laserdriver and VCSEL lasers are placed on the same substrate, and optimizedfor desired RF performance. FIGS. 28A-C illustrate an embodiment of acompact VCSEL laser driver assembly for multi-wavelength LIDAR of thepresent teaching. FIG. 28A illustrates a top-view 2800 of an embodimentof a compact VCSEL laser driver assembly of the present teaching. FIG.28B illustrates a side-view 2830 of the embodiment of the compact VCSELlaser driver assembly of FIG. 28A. FIG. 28C illustrates a bottom-view2850 of the embodiment of the compact VCSEL laser driver assembly ofFIG. 28A.

Referring to FIGS. 28A-28C, the array of VCSELs 2802 is placed on oneside of a substrate 2804. The driver chips 2806 are placed on theopposite side. A multi-layer substrate 2804 can be used to route thelaser driver signals from the drivers 2806 to the lasers in the array ofVCSELs 2802. The multi-layer substrate 2804 could be a printed circuitboard, a ceramic substrate, or on a flexible multi-layer circuit. Oneskilled in the art will appreciate that other substrate materials canalso be used.

FIG. 29 illustrates another embodiment of a compact VCSEL laser driverassembly 2900 for a multi-wavelength LIDAR system according to thepresent teaching. The laser drivers 2902 are located on the samesubstrate 2904 and on the same side as the VCSEL array 2906. An assembly2900 with nine quad laser driver IC's is positioned on the substrate forthirty-six VCSEL lasers in the VCSEL array 2906.

FIG. 30 illustrates a system block diagram of an embodiment of a compactVCSEL laser driver assembly 3000 for a multi-wavelength LIDAR of thepresent teaching. In this embodiment, the pulse generation chain 3002 isgenerated locally on the same carrier of the VCSEL assembly 3004. Thepulse generation chain 3002 comprises a pulse controller 3006, memory3008, pulse pattern generator 3010, and a laser driver 3012. The laserdriver 3012 is connected to a VCSEL laser 3014, as shown. In someembodiments, the laser driver is connected to a common contact used todrive multiple VCSEL lasers. In some embodiments, pulse shapes might bestored in a local memory or generated by a combination of the controllerand pattern generator.

The system processor 3016 is connected via a digital input/outputconnection 3018. The system processor 3016 generates a set ofinstructions that controls the laser instructing the laser to fire andfor how long. These instructions will determine the pattern type. But,the pattern generation and biasing of the lasers is done locally on theVCSEL assembly. Generating the laser driver pulse patterns locally onthe VCSEL assembly greatly simplifies the required interface to theoverall LIDAR system. In some embodiments, the pulse controller 3006,memory 3008, pulse pattern generator 3010 and laser driver 3012functions are all contained within a single IC package. In variousembodiments, the VCSEL devices can be hermetically packaged ornon-hermetically packaged.

FIG. 31 illustrates an embodiment of a multi-wavelength optical powermonitor for multi-element multi-wavelength LIDAR systems of the presentteaching. This multi-element multi-wavelength LIDAR system utilizes thesame illuminator projection element as in the multi-wavelength LIDARsystem described in connection with FIG. 14 , but with additionalelements to enable optical power monitoring. A partial mirror 3102reflects a portion of the optical beams 3104 at a point in the opticalsystem where light beams from a plurality of lasers 3106 will bereflected. In some embodiments, the placement of the partial mirror 3102is at a single point in the optical system. In other embodiments,reflected light is sampled at more than one point in the system usingmultiple reflective elements. The multiple reflection elements can bepartial mirrors. The multiple reflection elements can also project thebeam.

The reflected light is directed to a set of monitor photodetectors 3108,3310, which are each sensitive to only one wavelength of light. Themonitor photodetectors 3108, 3110 can simple be a notch filterpositioned in front of individual broadband photodiodes. One aspect ofthe multi-wavelength LIDAR of the present teaching is the ability tohave simultaneous and independent operation of lasers of differentwavelengths. This allows monitoring optical power of the two wavelengthsindependently, which improves the system capabilities.

There is typically a location within the optical system at which thebeams largely overlap. In the monitored LIDAR illuminator of FIG. 31 ,the overlap occurs after the optical device 3112. A reflecting elementand a partial mirror 3102 mounted on a transparent window 3114 can bepositioned in this overlap position. The window 3114 makes for aconvenient mounting surface, and could be replaced by any mechanicalstructure that substantially passes the light. The reflecting elementand the partial mirror 3102 reflects a portion of the light, which istypically less than 5% of the total optical power. The reflectingelement and partial mirror 3102 can be mounted on either side of thewindow 3114. In addition, this reflecting element and partial mirror3102 is shown reflecting the light of the different wavelengths in twodirections. Separate detectors 3108, 3110 are used for detecting andmonitoring the light from each wavelength used. It may also be the casethat the reflecting element and partial mirror 3102 reflects the lightfrom both wavelengths in one direction, while the light is still sampledseparately using separate photodiodes and appropriate wavelengthfiltering.

The reflected light detected within the illuminator can also be used toprovide additional control of the laser current bias. A laser diode hasa range of operating bias currents. Lasers systems, including LIDARsystems, are often operated in closed loop fashion where the receivedphotodiode current from the monitor diode serves as an input to a biascontrol loop. The LIDAR system will be able to react to changes insystem stability, such as temperature or mechanical shifts, and tomaintain an improved output power stability by monitoring andmaintaining a constant value of the monitor photodiode current, which isa largely linear function of the incident power. Also, this monitoringand control of the laser bias can accommodate some amount of degradationof the laser over its lifetime without loss of optical power at systemlevel.

Multi-wavelength power monitoring also improves the system robustnessfor detecting whether a fault is caused by laser degradation or shiftsin optical performance. Multi-wavelength power monitoring providesredundancy if one set of wavelengths should fail. A partial or fullfailure in operation of one set of wavelengths would still allow theability for partial operation of the system using the other set ofwavelengths if the optical monitoring for each wavelength isindependent.

Another aspect of the present teaching is a LIDAR system that performsoptical monitoring. In some embodiments, the light received at eachdetector 3108, 3110 is monitored for parameters, such as laserwavelength, optical power, pulse timing, and pulse frequency. In theseembodiments, the receiver is not simply a photodiode detector asdepicted in FIG. 31 , but also includes optics that allows detection ofwavelength as well as optical power. In a LIDAR design where multiplewavelengths are used, particularly if the wavelengths are close inabsolute value, it may be desired to monitor their absolute or relativevalues in order to ensure that the system parameters are as intended.Various methods of monitoring either absolute wavelength of the laser,or the relative offset between lasers of different wavelength are knownin the art. Some of these methods use an etalon-based monitoring device.

Another feature of the multi-wavelength LIDAR systems of the presentteaching is that wavelength differences of lasers positioned in an arraycan be used to improve collimation. In practice, a wafer of VCSELdevices is often manufactured such that the nominal wavelength is commonacross the full wafer, and the distribution of wavelengths within asmall portion of the wafer, as covered by a single VCSEL cluster is verysmall, likely less than 1 nm. The nominal VCSEL wavelength may not beable to be shifted across the wafer in some cases, depending on theVCSEL design approach, and manufacturing method employed. However, thereare some VCSEL manufacturing methods that allow different wavelengths tobe targeted for different apertures. In such a VCSEL, we can make use ofthe wavelength shift along with a wavelength sensitive optic to providebetter collimation than that achieved with non-wavelength sensitiveoptics.

FIG. 32 illustrates an embodiment of a VCSEL cluster layout 3200comprising laser apertures emitting different wavelengths according tothe present teaching. The VCSEL cluster layout 3200 contains thirteenVCSEL apertures 3202. This VCSEL cluster layout 3200 is chosen forsimplicity to better explain the method, whereas in actual practice adenser, more uniformly populated VCSEL cluster may be used. In thisembodiment, the wavelength of individual VCSEL apertures 3202 within thecluster shifts based on radial position. And, the wavelength emitted bythe VCSEL apertures 3202 along a particular radius is constant for allangles. In FIG. 32 , each VCSEL aperture 3202 is labeled with a numberindicating what position it is from the center VCSEL aperture.

FIG. 33 illustrates an embodiment of an illuminator comprising the VCSELcluster layout of FIG. 32 . The VCSEL cluster 3302 is positioned with awavelength sensitive optical device 3304 according to the presentteaching. The VCSEL cluster 3302 has wavelength varying radially, asdescribed in connection with FIG. 32 . In this case, the wavelengthsensitive optical device 3304 acts to deflect the optical beams 3306 byan amount dependent on their wavelength. The paths of the optical beams3306 projected by the optical device 3304 is a parallel set of paths.The deflection of the optical device 3304 thus results in a smallerdiameter collimated beam then can be achieved if all of the apertures inthe cluster were the same wavelength. In some embodiments, thewavelength sensitive optical device is a diffractive optic manufacturedsuch that the magnitude of deflection of an input beam of light dependson its wavelength.

In some embodiments, the VCSEL structure is configured and the VCSEL ismanufactured so that particular wavelengths within the cluster aredesired wavelengths. In some embodiments, the wavelength within theVCSEL cluster is changed as a function of radial position using a VCSELcluster that is configured to have a temperature gradient across thestructure. A typical VCSEL laser is sensitive to temperature with awavelength temperature coefficient of 0.1 nm/° C. A VCSEL cluster thathad a structure with a thermal gradient that varies radially would alsolead to shift of the VCSEL wavelength across the cluster, even if thedevices all emit the same wavelength at a constant temperature.

EQUIVALENTS

While the Applicant's teaching are described in conjunction with variousembodiments, it is not intended that the applicant's teaching be limitedto such embodiments. On the contrary, the Applicant's teaching encompassvarious alternatives, modifications, and equivalents, as will beappreciated by those of skill in the art, which may be made thereinwithout departing from the spirit and scope of the teaching.

1-61. (canceled)
 62. A multi-wavelength LIDAR system comprising: a) afirst array of laser emitters that generate a first array of opticalbeams having a first wavelength; b) a second array of laser emittersthat generate a second array of optical beams having a secondwavelength; c) a region of free space positioned in a path of the firstarray of optical beams and a path of the second array of optical beams,the region of free space interleaving the first array of optical beamsand the second array of optical beams at a distance from the first arrayof laser emitters; d) an optical receiver positioned to receive aportion of light from the first array of optical beams reflected at atarget plane and to receive a portion of light from the second array ofoptical beams reflected at the target plane, the optical receivergenerating at a first output a first plurality of wavelength signalscorresponding to the received portion of light from the first array ofoptical beams and generating at a second output a second plurality ofwavelength signals corresponding to the received portion of light fromthe second array of optical beams; and e) a controller having a firstinput that is electrically connected to the first and second outputs ofthe optical receiver, the controller generating a measurement pointcloud from the first and second plurality of wavelength signalsgenerated by the optical receiver, wherein an angular resolution of themeasurement point cloud comprises a first angular resolutioncorresponding to an emitter spacing of the first array of laser emittersand a second angular resolution corresponding to an emitter spacing ofthe second array of laser emitters.
 63. The multi-wavelength LIDARsystem of claim 62 wherein the target plane is at a distance from thefirst array of laser emitters that is greater than the distance from thefirst array of laser emitters that the region of free space interleavesthe first array of optical beams and the second array of optical beams.64. The multi-wavelength LIDAR system of claim 62 wherein at least oneof the first array of laser emitters and the second array of laseremitters comprises a one-dimensional array.
 65. The multi-wavelengthLIDAR system of claim 62 wherein at least one of the first array oflaser emitters and the second array of laser emitters comprises atwo-dimensional array.
 66. The multi-wavelength LIDAR system of claim 62wherein at least one of the first array of laser emitters and the secondarray of laser emitters comprises a VCSEL array
 67. The multi-wavelengthLIDAR system of claim 62 wherein the first array of laser emitters andthe second array of laser emitters are positioned in a single plane. 68.The multi-wavelength LIDAR system of claim 67 wherein the first array oflaser emitters and the second array of laser emitters are offset in thesingle plane
 69. The multi-wavelength LIDAR system of claim 68 whereinthe offset comprises a 10-mm offset.
 70. The multi-wavelength LIDARsystem of claim 62 wherein the first array of laser emitters and thesecond array of laser emitters are positioned at an angle with respectto each other.
 71. The multi-wavelength LIDAR system of claim 62 furthercomprising a first lens that projects the first array of optical beamsinto the free space region.
 72. The multi-wavelength LIDAR system ofclaim 71 further comprising a second lens that projects the second arrayof optical beams into the free space region.
 73. The multi-wavelengthLIDAR system of claim 71 wherein the first lens comprises a lenslet. 74.The multi-wavelength LIDAR system of claim 73 wherein the lensletcomprises a lenslet integrated with the first array of laser emitters.75. The multi-wavelength LIDAR system of claim 71 wherein the first lenscomprises a shared lens system.
 76. The multi-wavelength LIDAR system ofclaim 62 wherein the distance from the first array of laser emittersthat the region of free space interleaves the first array of opticalbeams and the second array of optical beams comprises a distance of lessthan 100 mm
 77. The multi-wavelength LIDAR system of claim 62 whereinthe first and second angular resolution are a same angular resolution.78. The multi-wavelength LIDAR system of claim 62 wherein the first andsecond angular resolution are a different angular resolution.
 79. Themulti-wavelength LIDAR system of claim 62 wherein the angular resolutionof the measurement point cloud comprises an angular resolution that islower in one direction than an angular resolution in a perpendiculardirection.